1. Field of the Invention
The present invention relates to a crosslinkable or crosslinked rubber composition which comprises a diene elastomer having a mass content of cyclic vinyl units of greater than 15%, and which may be used to form a tire tread. The invention also relates to such a tread and to such a tire incorporating said tread.
2. Description of Related Art
A conjugated diene monomer, such as butadiene, may be incorporated into a polymer obtained anionically in the form cis-1,4, trans-1,4 or -1,2 (vinyl linkages).
Anionic polymerization reactions are conventionally initiated by an organolithium compound in the presence of a hydrocarbon solvent and one or more conjugated diene monomers. The elastomers produced by these reactions have a low average mass content of vinyl linkages, generally being between 8% and 15%. In order to obtain elastomers having a given glass transition temperature (Tg), it is known to add to the polymerization medium a polar agent, which also has the effect of significantly increasing the average content of these linkages. This content may then reach 90%, for example.
US patent specifications U.S. Pat. Nos. 5,620,939, 5,906,956 and 6,140,434 disclose anionic polymerization processes which are performed batchwise, are intended to increase the content of vinyl linkages in a diene elastomer which may be a polybutadiene, a styrene-butadiene copolymer or a polyisoprene. Furthermore, these documents indicate that the diene elastomers so obtained are usable in rubber compositions for tire treads, whether associated with carbon black or with silica. These batch polymerization processes are carried out at a temperature of between 30 and 100° C. and they in particular involve the use of catalytic systems comprising:
a polar agent comprising one or more heteroatoms, such as tetrahydrofuran (THF) or tetramethylethylenediamine (TMEDA),
an organolithium initiator, such as n-butyllithium, and
an alkali metal salt of an aliphatic or alicyclic alcohol, such as sodium tert.-amylate or sodium mentholate.
More precisely, in its Examples, document U.S. Pat. No. 5,620,939 discloses molar ratios of [(salt:initiator), (salt:polar agent), (polar agent:initiator)] which are equal to [1; 0.33; 3], [2; 0.66; 3], [3; 1; 3], [0.5; 0.17; 3], [1; 0.5; 2] and [1; 0.2; 5].
In its Examples, document U.S. Pat. No. 5,906,956 discloses the following values for said molar ratios: [2; 1; 2], [0.5; 1; 0.5], [0.25; 0.08; 3], [0.5; 0.17; 3], [1; 0.33; 3], [1; 0.66; 3], [3; 1; 3], [0.5; 0.17; 3], [1; 0.5; 2] and [1; 0.2; 5].
In its Examples, document U.S. Pat. No. 6,140,434 discloses the following values for said molar ratios: [0.25; 0.03; 8], [0.25; 0.05; 5], [0.25; 0.08; 3], [0.25; 0.25; 1], [0.25; 0.5; 0.5], [0.15; 0.05; 3], [0.5; 0.17; 3], [1; 0.33; 3] and [0.25; 0.13; 2].
Independently of raising the content of vinyl linkages in these elastomers, the attempt has in the past been made to form cyclic structures of the vinylcyclopentane type in diene elastomers obtained anionically.
One solution for forming these cyclic structures involves performing post-polymerization reactions, for example by treating the elastomers with Lewis acids.
Another solution may involve forming these cyclic structures directly during anionic polymerization by performing said polymerization continuously with a low monomer concentration in the reaction medium and using a specific catalytic system.
In its examples of embodiment, US patent specification U.S. Pat. No. 3,966,691 describes a process for cyclizing butadiene in the polymerization medium, which involves using a catalytic system comprising an initiator consisting of n-butylsodium and an activator consisting of tetramethylethylenediamine (TMEDA). A lithium alkoxide is optionally added to the polymerization medium to accelerate polymerization.
Polymerization is performed continuously at a temperature of between −20° C. and 150° C., and the polybutadienes obtained have a mass content of cyclic structures of greater than 40%, but a number-average molecular weight of no more than 2,000 g/mol.
The articles “G. Quack, L. J. Fetters, Macromolecules, Vol 11, no. 2, pp. 369-373, (1978)” and “A. R. Luxton, M. E. Burrage, G. Quack, L. J. Fetters, Polymer, Vol 22, pp. 382-386” also describe continuous cyclization processes in the polymerization medium, which involve using catalytic systems comprising an organolithium initiator, such as n-butyllithium, and a polar agent consisting of TMEDA (reference may be made to page 383, right hand column of the last-cited document for a description of the cyclization mechanism).
The polymerizations are performed at temperatures of between 30 and 90° C. and the polybutadienes obtained have a mass content of vinylcyclopentane cycles of at least 18%, but a number-average molecular weight of no more than 5,000 g/mol.
One major drawback of these polymerization processes is thus the very low value of the number-average molecular weight of the diene elastomers obtained, which means that these elastomers cannot be used to form the elastomeric matrix of a rubber tire tread composition.